Current strategies for the prevention of graft rejection after transplantation are based on the use of broad acting immunosuppressive agents such as cyclosporin A (CsA), FK506 and corticosteroids. These drugs must often be taken over long periods of time and therefore increase the risk of serious infections, nephrotoxicity, and cancer. In addition, not all patients can tolerate high doses of these immunosuppressive agents, often resulting in graft rejection or graft-versus-host disease (GVHD). Optimal prevention of graft rejection should be based on the induction of specific tolerance to the donor tissue. Thus, the ideal drug for prevention of transplant rejection should induce clonal unresponsiveness, or anergy, of donor-reactive T cells, without the need for long-term immunosuppression. Anergy is thought to be the result of intercellular signaling after interaction between the T-cell receptor (TCR) and the peptide-presenting major histocompatibility complex (MHC) antigen in the absence of a "costimulatory" signal. Mueller, D. L. et al., Annu. Rev. Immunol. (1989) 7:445. This costimulatory signal is normally provided by the cell surface of antigen-presenting cells (APCs). Hawrylowicz, C. M. et al., J. Immunol. (1988) 141:4083; and Springer, T. A. et al., Annu. Rev. Immunol. (1987) 5:223.
T Cell Activation
T cells play an important role during the normal in vivo immune response. They are involved in cytotoxicity, delayed type hypersensitivity, and T cell-dependent antibody production by B cells. Furthermore, T cells can produce a wide variety of lymphokines such as interleukin-2 (IL-2), tumor necrosis factor alpha (TNF-.alpha.), lymphotoxin, gamma interferon (IFN-.gamma.), and granulocyte macrophage colony stimulating factor (GM-CSF).
The activation of T cells is the result of ligand-receptor interactions. Under physiological conditions, the TCR/CD3 complex binds to antigenic peptides presented by the MHC molecules of APCs. The TCR/CD3 complex plays two roles in antigen-induced activation. First, it recognizes a specific antigen in the context of an antigen-presenting MHC molecule. Then, the recognition event is transmitted through the plasma membrane by a signalling mechanism. However, binding of antigen to the TCR alone is not sufficient for maximum T cell activation. A number of other accessory molecules on the surface of the T cell are known to play important roles in adhesion or signalling or both. For instance, the CD2 molecule on T cells can bind to LFA-3 on APCs, but it has also been shown that binding of antibodies to CD2 can augment the signals provided by the TCR/CD3 complex. Other ligand pairs involved in T cell activation are LFA-1/ICAM-1, CD4/MHC-class II antigen, VLA-4/VCAM, and, most importantly, CD28/B7.
The CD28 and B7 Antigens
The likely candidate for the costimulatory signal that determines whether TCR-stimulation leads to full T cell activation or to T cell anergy is that generated by interaction of CD28 on the T cells with B7 on APCs. It has been demonstrated in vitro that cross-linking of the CD28 molecule can rescue T cells from becoming anergic. Harding, F. A. et al., Nature (1992) 356:607. CD28 is a homodimeric transmembrane glycoprotein with an apparent molecular mass of 44 kDa and is a member of the immunoglobulin superfamily (Aruffo, A. & Seed, B. PNAS (USA) (1987) 84:8573). The CD28 molecule is expressed on approximately 95% of CD4-positive T cells and 50% of CD8-positive T cells. Costimulation of T cells with monoclonal antibody to the TCR/CD3 complex and CD28 results in greatly enhanced T cell activation. Thompson, C. B. et al., PNAS (USA) (1989) 86:1333-1337; June, C. H. et al., J. Immunol. (1989) 143:153-161; and Lindsten, T. et al., Science (1989) 244:339-343. This effect apparently involves stabilization of mRNA for several lymphokines, including IL-2, resulting in a greatly enhanced production of these lymphokines. June, C. H. et al., supra; and Lindsten, T. et. al., supra. Furthermore, a CD28-responsive element has been demonstrated in the enhancer of the IL-2 gene, suggesting that there is also regulation at the transcriptional level. Fraser, J. D. et al., Science (1991) 251:313 and Verwey, C. L. et al., J. Biol. Chem. (1991) 266:14179-14182. Certain models of T cell activation mediated by CD28 have been reported to be relatively resistant to inhibition with CsA. June, C. H. et al., Mol. Cell. Biol. (1987) 7:4472-4481.
B7 is a monomeric transmembrane glycoprotein with an apparent molecular mass of 45-65 kDa and is, like CD28, a member of the immunoglobulin superfamily. Freeman, G. J. et al., J. Immunol. (1989) 143:2714-2722. Moreover, B7-expressing CHO cells are able to synergize with TCR stimulation, resulting in IL-2 secretion and T cell proliferation. Linsley, P. S. et al., J. Exp. Med. (1991) 137:721-730; and Gimmi, C. D. et al., PNAS (USA) (1991) 88:6575. B7 also binds to a recombinant fusion protein of the CTLA-4 molecule. Linsley, P. S. et al., J. Exp. Med. (1991) 174:561-569. CTLA-4, too, is a member of the immunoglobulin superfamily, and the cytoplasmic regions of CTLA-4 and CD28 show significant homology. Harper, K. et al., J. Immunol. (1991) 147:1037. The B7 molecule is expressed on activated B cells (Freeman, G. J. et al., supra), monocytes stimulated with IFN-.gamma. (Freedman, A. S. et al., Cell. Immunol. (1991) 137:429-437), and isolated peripheral blood dendritic cells (Young, J. W. et al., J. Clin. Invest. (1992) 90:229-237). Immunohistochemical studies have shown that the B7 molecule is also constitutively expressed in vivo on dendritic cells in both lymphoid and non-lymphoid tissue. Vandenberghe, P. et al., "International Immunology" (1993).
In vivo, the B7 antigen is involved in T cell activation during transplant rejection. Lenschow and co-workers have used a soluble fusion protein of human CTLA-4 and the immunoglobulin G1 Fc region (CTLA4Ig), which strongly binds to both mouse and human B7, to prevent rejection of human pancreatic islets after transplantation in mice (Lenschow, D. J. et al., Science (1992) 257:789-792). Here CTLA4Ig blocks rejection of a xenoantigen (an antigen foreign to the species from which the T cell is derived).
Molecules that interfere with the interaction between the B7 and CD28 antigens are known in the art. The soluble CTLA4-Ig fusion protein is known to partially block this interaction. Linsley, P. S. et al., J. Exp. Med. (1991) 74:561. Anti-CD28 antibodies are also known to block this interaction. Furthermore, anti-B7 antibodies are known in the art. Yokochi, T. et al., J. Immunol. (1982) 128:823; Freedman, A. S. et al. J. Immunol. (1987) 139:3260; Valle, A. et al. Immunol (1990) 69:531. Commonly owned, co-pending U.S. application Ser. No. 07/910,222 describes the generation of one such monoclonal antibody (mAb) called B7-24.
However, nothing in the art relates to (1) the use of anti-B7 antibodies to cause T cell anergy and thereby prevent or treat graft rejection or GVHD, or (2) the use of molecules that bind to the B7 antigen in conjunction with other immunosuppressive agents to cause T cell anergy and thereby prevent or treat graft rejection or GVHD.